1. Field of the Invention
The present invention relates to a lubricating oil blend having resistance to ionizing radiation. More particularly, the invention relates to an oil blend resistant to ionizing radiation, comprising a mixture of phenoxyphenoxydiphenyl and a monoalkyldiphenyl ether or dialkyldiphenyl ether. This oil has a pour point of 0.degree. C. or below and has a G-value of 0.1 or lower for the evolution of decomposition gases under irradiation with ionizing radiation. The term "G-value" used here represents the number of gas molecules liberated per 100 eV of absorbed energy in 1 gram of oil.
2. Description of the Prior Art
With the recent rapid increase in the use of machines under irradiation with ionizing radiation, there has arisen a demand for developing lubricating oils that have high resistance to ionizing radiation in addition to the properties possessed by ordinary lubricants. Petroleum base lubricating oils in current use do not meet this requirement. If they are used as lubricants for machines exposed to a high dose rate of radiation, they are soon decomposed to form either a gas or a solid which lacks any lubricating properties. It has also been reported in the art that if much gas is evolved, fire, metallic corrosion and/or vapor lock in the hydraulic system of machines may possibly be caused.
These problems are caused by the decomposition of the lubricant and the resulting increase in viscosity, total acid number and gas evolution. For example, 350 neutral oil has a G-value of 1.4 for the evolution of decomposition gases; when 1,000 ml of this oil is irradiated with 1 MR of gamma rays, it is decomposed to form 290 ml of decomposition gases. The lower the G-value for the evolution of decomposition gases, the more suitable the lubricating oils for use in a radiation field. If they have a G-value of 0.1, their service life can be extended by almost 10 times the life of petroleum-based lubricants. Conventionally, condensed polycyclic aromatic compounds, polyphenyls and polyphenyl ethers are known to have high resistance to ionizing radiation. However, most of these lubricating oils are available as solid or highly viscous liquid at room temperature or have pour points higher than 0.degree. C. Therefore, they cannot be effectively used, especially in winter, without equipping the lubricating system with a heater.
U.S. Pat. No. 3,130,162 discloses o-(m-phenoxyphenoxy)diphenyl which has a melting point of 50.degree.-51.degree. C., and said patent does not disclose said diphenyl to be a lubricating oil resistant to ionizing radiation.
U.S. Pat. No. 3,203,997 discloses aromatic substituted polyethers, such as bis[p-(p-.alpha.-cumylphenoxy)phenyl]ether having radiation-resistant characteristics, but said ether is solid at room temperature and exhibits no compatability with m-(m-phenoxyphenoxy)diphenyl.
U.S. Pat. No. 3,471,574 discloses m-(m-phenoxyphenxoy(biphenyl which has a pour point of 2.5.degree. C., and which can be blended with polyphenyl ether compounds. However, as indicated in U.S. Pat. No. 3,203,997, bis[p-(p-.alpha.-cumylphenoxy)phenyl]ether exhibits no compatability with m-(m-phenoxyphenoxy)diphenyl, so the latter statement is not completely accurate.
In Japanese Patent Laid-Open (Kokai) No. 60598/80, the present inventors showed that a polyphenyl ether mixture containing phenoxyphenoxydiphenyl can be easily produced by reacting an alkali metal salt of a mixture of phenol and phenylphenol with m-dihalogenobenzene in the presence of a copper catalyst under the conditions for the Ullmann reaction. The inventors continued their studies on the resistance of various lubricating oils to radioactive rays and showed that m-(m-phenoxyphenoxy)diphenyl exhibited an extremely high resistance to ionizing radiation applied together with oxygen bubbling [H. Nakanishi, K. Arakawa, N. Hayakawa, S. Machi and T. Yagi; preprint for the 25th Anniversary Tokyo Seminar, Japan Society of Lubrication Engineers, A 24 (1980) 161]. They also showed that the m-(m-phenoxyphenoxy)diphenyl had a G-value of as low as 0.005 for the evolution of decomposition gases under irradiation. However, this oil does not have an adequately low pour point. On the other hand, monoalkyldiphenyl ethers or dialkyldiphenylethers have good properties for use as lubricating oils (i.e. low pour point, high flash point and high viscosity index). They also have high resistance to heat and oxidation. However, as compared with phenoxyphenoxydiphenyl, these ethers are somewhat low in resistance to ionizing radiation as applied in the presence of bubbled oxygen and they have a G-value of 0.4-0.6 for the evolution of decomposition gases.